It'd be fair to say that there are styles of play, sub-genres, and character & campaign concepts that are unsupported or inadequately supported, by the standard game
That's what 3PP covers. WotC is building the basic game and has their focus. They are currently successful.
My advice is look at the 3PP for those niche sub-genres, styles of play, and character concepts. Then pick one a person likes and buy it. Supporting those products demonstrates a market for those products. It's the vote-with-your-wallet method and those designers are certainly going to appreciate the support for their work.